In a 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) system, a User Equipment (UE) connected to a Base Station (BS) measures the Reference Signal Received Power (RSRP) of the BS, evaluates a downlink channel state using the RSRP, and reports the downlink channel state to the BS, periodically or in an event-triggered manner. The downlink channel state information (or downlink channel information) may include a Channel Quality Indication (CQI), a Precoding Matrix Index (PMI), and a Rank Indication (RI). The UE transmits all or some of the CQI, PMI and RI according to transmission mode.
The BS determines time and frequency resources and a Modulation and Coding Scheme (MCS) for data transmission to the UE based on the received downlink channel state information.
The CQI is determined based on the quality of a signal received at the UE. In general, the CQI is determined based on the measurement of a Reference Signal (RS) received at the UE.
The UE may report the channel state information periodically (periodic reporting), or aperiodically upon request of the BS (aperiodic reporting).
In case of aperiodic reporting, when the BS requests reporting of channel state information to the UE, the UE transmits the channel state information to the BS on a Physical Uplink Shared CHannel (PUSCH).
In case of periodic reporting, when the BS notifies the UE of a transmission period of channel state information and a transmission offset for use in the transmission period, the UE transmits channel state information on a Physical Uplink Control CHannel (PUCCH) during the transmission period. If there is uplink data to be transmitted together with the channel state information in a subframe, the UE may transmit both the channel state information and the data on a PUSCH.
Depending on CQI feedback types and PMI feedback types, four reporting modes are available for periodic reporting. Table 1 below illustrates the four periodic reporting modes of channel state information.
TABLE 1PMI Feedback TypeNo PMISingle PMIPUCCH CQIWideband(widebandMode 1-0Mode 1-1Feedback TypeCQI)UEMode 2-0Mode 2-1Selected(subbandCQI)
The CQI feedback types are WideBand (WB) CQI and SubBand (SB) CQI, and the PMI feedback types are No PMI and Single PMI depending on whether a PMI is transmitted or not. A WB CQI refers to the CQI of a total frequency band available to the UE, whereas an SB CQI refers to the CQI of part of the total frequency band. The UE may or may not transmit a PMI to the BS. The UE may receive information about a transmission period and offset of channel state information by higher-layer signaling (Radio Resource Control (RRC) signaling).
FIG. 1 illustrates a method for transmitting channel information at a UE, when a transmission period and offset of channel state information is 5 and 1, respectively.
Referring to FIG. 1, if the transmission period of channel state information is 5, the UE transmits channel state information in every 5 subframes. For a transmission offset of 1, the transmission of the channel state information starts in the first subframe counted from subframe 0 in an ascending order of subframe indexes, that is, in subframe 1. Thus, the UE transmits the channel state information on a PUCCH in subframes 1 and 6.
Specifically, the UE transmits an average CQI of subframes 1 to 5 or the CQI of any of subframes 1 to 5 in subframe 6. Or the UE may transmit an average CQI of an arbitrary period selected from subframes 1 to 5.
The indexes of subframes are given as combinations of the number of system frames, of and the indexes of 20 slots per system frame, ns. Since one subframe includes two slots, the index of a subframe may be expressed as 10×nf+floor(ns/2).
The UE transmits only a WB CQI or both a WB CQI and an SB CQI. FIG. 2 illustrates resources along a frequency axis.
Referring to FIG. 2, a total frequency band includes 16 Resource Blocks (RBs). The total frequency band is divided into two Bandwidth Parts (BPs), each BP having two SBs. Thus, each SB includes four RBs. The number of BPs and the size of each SB depend on the number of RBs included in a system frequency band, and the number of SBs per BP is determined according to the number of RBs, the number of BPs, and the size of each SB.
In a CQI feedback type of transmitting both a WB CQI and an SB CQI, the WB CQI is first transmitted in a CQI transmission subframe, the CQI of an SB in the better state between SB0 and SB1 in BP0 is transmitted in the second CQI transmission subframe, and the CQI of an SB in the better state between SB0 and SB1 in BP1 is transmitted in the third CQI transmission subframe.
Specifically, the CQIs of the two BPs are sequentially transmitted after transmission of the WB CQI. During the time interval between two subframes carrying WB CQIs, the CQIs of the BPs may be sequentially transmitted once to four times. For example, if the CQI of each BP CQI is transmitted once during the time interval between two subframes carrying WB CQIs, CQIs are transmitted in the order of WB CQI, BP0 CQI, BP1 CQI, and WB CQI. If the CQI of each BP CQI is transmitted four times during the time interval between two subframes carrying WB CQIs, CQIs are transmitted in the order of WB CQI, BP0 CQI, BP1 CQI, BP0 CQI, BP1 CQI, BP0 CQI, BP1 CQI, BP0 CQI, BP1 CQI, and WB CQI.
FIG. 3 illustrates a case where both a WB CQI and an SB CQI are transmitted.
In the illustrated case of FIG. 3, a CQI transmission period is 5, a CQI transmission offset is 1, and the CQI of each BP is transmitted once during the time interval between two subframes carrying WB CQIs.
The number of sequential transmissions of the CQI of each BP between two subframes carrying WB CQIs is indicated by higher-layer signaling.
If a PMI is also transmitted, a PMI and a CQI are transmitted together. In the presence of a PUSCH to carry uplink data in a transmission subframe, the PMI and the CQI are transmitted along with data on the PUSCH.
With regards to RI transmission, the BS notifies the UE of a transmission period of an RI and an offset for the transmission period. The transmission period of an RI is given as a multiple number of the transmission period of a WB CQI, and the RI transmission offset is a value relative to a CQI transmission offset. For example, if a CQI transmission offset is 1 and an RI transmission offset is 0, the CQI and the RI have an identical offset. The RI transmission offset is 0 or a negative-signed value.
FIG. 4 illustrates a case where an RI transmission period being a multiple of a WB CQI transmission period is 1 and an RI transmission offset is −1.
Because the RI transmission period is one multiple of the WB CQI transmission period, the RI transmission period is equal to the WB CQI transmission period. Given an RI offset of −1, an RI is transmitted in subframe 0 in FIG. 3.
Heterogeneous systems with various types of small base stations such as relays and femtocells as well as legacy BSs have recently been discussed. Inter-cell coordinating communication such as simultaneous transmission of a signal to a single UE from a plurality of coordinating BSs or suspended transmission of neighbor cells to reduce interference at a cell boundary is another recent study area. In a heterogeneous system or inter-cell cooperative communication, a UE may experience an interfering link whose configuration and state may greatly change in time and frequency.
However, the conventional method for transmitting channel state information degrades system performance because a UE measures channel quality along the time and frequency axes with no regard to an interfering link and reports the channel quality to a BS.